For many uses of railroad hopper cars it is desirable to pressurize the car so that, for example, dust and dirt in the air are unable to get inside the car and contaminate the contents, such as plastic pellets covered with a layer of inert gas. Hatch covers for pressurized cars typically have a nominal diameter (measured by the I.D. of the "coaming" or hatch) of 20 inches. Using a cover of that size, under pressure, results in problems in the attempt to effectively seal the hatch.
Conventionally, the covers have been cast aluminum or cast stainless steel for rigidity and cleanliness, held down by three or more threaded hand wheels. The use of three or more hand wheels is an attempt to eliminate the problem of uneven sealing pressure (a good seal adjacent the hand wheels and a poor seal between the wheels). To improve the sealing capability, the covers were made thicker and more rigid, thus increasing their material cost. In an effort to reduce the excessive material cost and weight of the cover, some prior art hatch covers were made from fiber reinforced plastic (FRP), but it was found not to be economically feasible to make FRP covers which would be rigid enough to prevent leakage from the hatch. By necessity, therefore, railroad car manufacturers have continued to make the hatch covers from aluminum or stainless steel and the continued use of the hand wheel sealing apparatus involves additional problems, of the type normally anticipated where it is necessary to constantly run a threaded member into and out of another member having internal threads. Not only do the threaded portions corrode, jam and cross-thread, but also a considerable amount of time, skill and care are required to run-down the series of threaded hand wheels so that the seal pressure is as uniform as is possible with such a cover and lock arrangement, while not damaging the gasket. It is most advisable to tighten the wheels alternately and in stages as is done with the lug-nuts for an automobile wheel.
The concern over the prevention of dirt getting to the contents of the hopper car is well illustrated by the procedure followed when the contents are unloaded. Before the underside of the car is opened to permit removal of the contents, filters are placed over the tops of the hatches to prevent dirt from being sucked in by the partial vacuum created as the material is dumped from the bottom of the hopper. Therefore, it can readily be appreciated that the ability to lock and unlock the hatch cover quickly is of importance to the railroads, shippers and receivers.
In U.S. Pat. No. 3,821,935, granted July 2, 1974, invented by Franklin P. Adler and assigned to Pullman Transport Leasing Company, a hatch cover is provided with a handwheel having a threaded shaft extending through the hatch cover. A hub is threaded onto the shaft having a plurality of radially extending locking arms which engage fixed lugs on the hatch coaming. A guide plate depending from the cover includes a cam slot which receives a follower extending from the hub. Rotation of the handwheel causes rotation and vertical movement of the locking arms into engagement with a lip on the coaming. To open the cover the handwheel is rotated in the opposite direction until the locking arms clear the locking lugs. The cover is then pivoted about a hinge to the open position.
This construction is disadvantageous because of the difficulty of obtaining a satisfactory seal between the threaded shaft and the cover and the large number of turns of the handwheel required to move the locking arms between open and closed position and because of cost. Furthermore, the internal friction between the threaded shaft and hub may result in some contamination by lubricant and/or ground metal. Also the screw is inside where it can (and does) contaminate the load with corrosion and lubricant.
Also the force urging the seal into engagement with the coaming is dependent upon the number of turns applied by the operator. If an insufficient number of turns is applied by the operator, the cover may not remain properly seated in transit.
An improved version of the hatch cover disclosed in U.S. Pat. No. 3,821,935 is disclosed in U.S. Pat. No. 4,073,239 invented by Franklin P. Adler and assigned to Apex Railway Products Co., however, many of the same problems with the cover described in U.S. Pat. No. 3,821,935 remain.
Another approach to the hatch cover construction is shown in U.S. Pat. No. 4,132,327 invented by Ronald D. Van Dyke, et al., and assigned to ACF Industries, Inc. While this construction eliminates some of the problems with the cover described in the aforesaid patents, the same problems respecting center bolt leakage and corrosion remain and the leaf spring retainer substituted for the massive (and expensive) spider of U.S. Pat. No. 3,821,935 is not rugged enough for long term use.